In recent years, there has been actively developed a steering apparatus for a vehicle which can impart a proper feedback to a steering wheel at all times irrespective of the traveling environment of the vehicle; in particular, even when the vehicle travels on a snowy road, an icy road, or a like road which provides a small road-surface reaction force. For example, Japanese Patent Application Laid-Open (kokai) No. 2002-211427 discloses an electric power steering control apparatus which imparts proper reaction torque to the steering wheel in accordance with the travel environment. This electric power steering control apparatus includes a steering angle sensor for detecting a steering angle of the steering wheel, a reaction torque sensor for detecting reaction torque of the steering system, and a superimposing reaction torque calculation section which calculates a superimposing reaction torque in the returning direction of the steering wheel by multiplying, by a gain, the steering angle detected by means of the steering angle sensor. The apparatus controls the gain so as to decrease the superimposing reaction torque when the reaction torque of the steering system is large, and increase the superimposing reaction torque when the reaction torque of the steering system is small.
However, the conventional electric power steering control apparatus is designed such that when the driver performs a proper steering operation, the apparatus imparts to the steering wheel a proper feedback; i.e., a superimposing reaction torque in response to a small reaction torque. There, in a case where the driver performs an unreasonable steering operation or carelessly performs an excessive steering operation in a travel environment where a small reaction force is provided by the road surface, the side slip angle of the steerable wheels increases, with a resultant sharp drop in the self-aligning torque of the steerable wheels, possibly resulting in failure to obtain the small reaction torque from the road surface. In a situation where such a small reaction torque is not obtained, generation of a proper superimposing reaction torque becomes difficult, with the result that the driver cannot obtain any feedback from the steering wheel during driving, and in some cases cannot drive the vehicle at will.
In order to cope with this problem, there is proposed a steering-reaction-force control apparatus which controls steering reaction generated against steering operation of the driver, as disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-154962. This steering-reaction-force control apparatus includes steering means (e.g., an electric power steering apparatus) capable of steering the steerable wheels (turnable wheels), an actuator capable of adding an operational force to the steering means, and side-slip angle detection means for detecting side slip angle of the steerable wheels (turnable wheels) in relation to the road surface. In order to impart a steering reaction to the steering means in response to generation of a side slip angle, the apparatus controls the actuator to generate the steering reaction in a direction in which the side slip angle of the steerable wheels (turnable wheels) is generated and to increase the steering reaction as the side slip angle increases.
According to this steering-reaction-force control apparatus, through detection of the side slip angle of the steerable wheels, the operation of the actuator can be controlled in consideration of a sharp drop in the self-aligning torque generated when the side slip angle of the steerable wheels is large. By virtue of this control, even in a travel environment in which the feedback or response to the rotation of the steering wheel decreases due to a sharp drop in the self-aligning torque, a steering reaction which increases with the side slip angle of the steerable wheels can be generated separately and applied to the steering wheel. Accordingly, even in a case where the side slip angle of the steerable wheels is large, the driver can feel a proper steering reaction.